This invention relates to a speech detector for determining the presence or absence of speech in a pulse-code-modulation (PCM) signal, more particularly to a speech detector with improved immunity to line faults. The invented speech detector is applicable in, for example, digital speech interpolation (DSI) equipment, digital channel multiplication equipment (DCME), and voice packetization equipment.
DSI, DCME, and voice packetization equipment utilize telephone channels efficiently by transmitting only those segments of a PCM-encoded signal in which speech is present, as determined by a speech detector. Prior-art speech detectors generally detect speech when the intensity level of the PCM signal, variously defined as the mean power, mean amplitude, or peak value of the signal over an interval of time, is above a certain threshold. To detect low-intensity speech, the speech detector may also test the zero-crossing count, defined as the number of sign changes of the PCM signal within the interval, and combine the intensity and zero-crossing detection results by OR logic. That is, speech is detected as present if either the intensity level or the zero-crossing count is over a respective threshold.
Line faults occur for a variety of reasons, ranging from equipment malfunctions to breakdown of transmission cables, between the site of origin of a signal and the input terminal of the speech detector, producing PCM signals that contain no meaningful speech information. To avoid the wasteful allocation of channels to or assembly of voice packets by such signals, when a line fault occurs, the speech detector should detect speech as absent.
Line faults, however, tend to create PCM signals with large direct-current offsets. For example, when a PCM signal is relayed by PCM primary-group multiplex equipment as stipulated in recommendation G.732, "Characteristics of Primary PCM Multiplex Equipment Operating at 2048 kbit/s," of the International Telegraph and Telephone Consultative Committee (CCITT), a line fault causes the transfer of an Alarm Indication Signal (AIS), as stipulated in Section 4.2 in the above recommendation, comprising eight-bit code words consisting of all one's (11111111). In the A-law PCM code used in PCM primary-group multiplex transmission systems, the code word 11111111 denotes an amplitude of approximately 2.6% the maximum amplitude that can be transmitted. Even a sinewave signal of this amplitude should easily exceed the intensity threshold for speech detection regardless of whether peak detection, mean-power detection, or mean-amplitude detection is used.
Existing speech detectors therefore tend to mistake line faults for the presence of speech, causing unnecessary allocation of channels or assembly of voice packets, thereby reducing channel utilization efficiency.